The present invention relates to phase cycling method in the SSFP pulse sequence of the gradient echo system, in which the phase shift of lateral magnetization developed in the TR by the gradient magnetic field prior to the next RF excitation, as well as to a magnetic resonance imaging apparatus using the phase cycling method.
SSFP pulse sequence for the gradient echo system in which the phase shift of lateral magnetization developed in the TR by the gradient magnetic field is roll back prior to the next RF excitation is in general referred to as FISP (fast imaging with steady-state precession) or FIESTA (fast imaging employing steady state acquisition). FISP is susceptible to the ununiform field, and is known to tend to develop a band-like low signal region on the image, called Band Artifact. The RF transmission phase of generic FISP is repeatedly at 0-180-0-180 (degrees) for an excitation, which develops a steady state (SSFP: steady state free precession).
A known solution for solving the band artifact in the FISP is phase cycling method, described in the reference 1.
For example, when 2Nex (number of addition), an image is obtained at the RF transmission phases 0-0-0-0(degrees), then another image is obtained at the RF transmission phases 0-180-0-180(degrees) to combine those two images to decrease the band artifact. The increment of each of RF transmission phases is at 0 and 180 degrees.
In a similar manner, when 3Nex, images are obtained at three RF transmission phases of 0-0-0-0 (deg), 0-120-240-0-120-240 (deg), 0-240-120-0-240-120 (deg) to combine to decrease the band artifact. In this case, the increment of RF transmission phases is at 0, 120, and 240 degrees.
When 4Nex, 0-0-0-0 (deg), images are obtained at four RF transmission phases of 0-90-180-270 (deg), 0-180-0-180 (deg), 0-270-180-90 (deg) to combine to decrease the band artifact. The increment of RF transmission phases is at 0, 90, 180, and 270 degrees.
FIGS. 7a, 7b, and 7c show schematic diagrams of the increment of RF transmission phase for each excitation in the conventional phase cycling method drawn on two-dimensional plane. As shown in the figures, in the conventional phase cycling method, one cycle of 360 degrees is uniformly divided to determine the increment of RF transmission phases, depending on the number of addition (Nex). Sampling a plurality of times by shifted increment of RF transmission phases may shift the position of developed band artifact in a plurality of images obtained, and the combination of thus sampled images may result in a decrease of affected band artifact.
Reference 1: JP-A-2004-121466
However, in the conventional phase cycling method, when the ununiformity of magnetic field is relatively not severe and Nex is small, for example in 2Nex, a significant band artifact on the center of image is developed, resulting in the problem that the band artifact is not always sufficiently decreased.